Basic Concepts

1. All activity everywhere occurs within and is controlled by feedback loops.
2. Feedback loops (and therefore all systems) consist of two and only two concepts: Levels (also called accumulations or stocks) and Rates (also called flows). Other equations in system dynamics software like auxiliaries, converters, constants and parameters are algebraically part of the Rate.
3. Recognize that models and simulations are a part of every day thinking. We all use models and simulations for every decision. These are called mental models. Formal simulation models make our assumptions explicit. They help to make our thinking precise, clarify ambiguities and identify gaps in logical structure.
4. Visualize and sketch behavior through time. See behavior unfold in the real world and identify model structures that generate the same behavior.
5. Identifying the purpose of a model and determining the boundary that includes the things that produce that behavior. Think of the closed boundary that contains the relationships that are essential for creating the behavior of interest.

Generic Structures for System Dynamics Modeling

Basic Model Structures

1. Linear Growth
2. Linear Decay
3. Compound Growth
4. Compound Decay
5. Goal Seeking

Generic Model Structures

1. Population = compound growth and decay with one stock
2. Implicit Goal Seeking = variation on goal seeking
3. External Resource Depletion = linear growth and goal seeking outflow
4. Convergent = variation on goal seeking
5. Nonlinear Delay = linear growth and implicit goal seeking outflow
6. Infection = goal seeking outflow from one stock combined with goal seeking inflow to another stock
7. Oscillations = linear and exponential growth combined with variations
8. Overshoot and Collapse = population linked with external resource depletion
9. Quadratic = link two linear models each with a biflow
10. Co-Flow = two models linked, each is linear growth and decay with one stock